1. Field of the Invention
The present invention relates to a new organic borate compound, a nonaqueous electrolyte using the compound, and a lithium secondary battery and an electric appliance both using the electrolyte; the invention relating especially to a new organic borate compound high in oxidation resistance, a nonaqueous electrolyte improved in oxidation resistance by use of the compound, a lithium secondary battery and an electric appliance, both improved in cycle life by use of the electrolyte, and various applications of the electric appliance.
2. Description of the Prior Art
Lithium secondary batteries each made up of positive and negative electrodes capable of occluding and releasing lithium, a nonaqueous electrolyte, and other components, are high in energy density per weight and volume and in voltage. They are, therefore hoped to be used as portable compact power supplies or as power supplies for electric automobiles. Since lithium secondary batteries have a high driving voltage of 3 V or more, they use a nonaqueous electrolyte wide in withstand voltage range. Compared with aqueous electrolytes, nonaqueous electrolytes have the drawbacks that they are low in electroconductivity and that a large portion of organic solvents suitable for an nonaqueous electrolyte are high in flammability (or low in flashing point).
For these reasons, researches for improving electroconductivity and, hence, the load characteristics of the battery, and researches for using a noncombustible or low-flammability organic solvent as the nonaqueous electrolyte for the battery, are taking place actively. An example of the former researches is reported in xe2x80x9cA Collection of Preprints, the 40th Forum on Batteries, pp. 453xcx9c454, (1999)xe2x80x9d, and an example of the latter researches is reported in xe2x80x9cA Collection of Preprints, the 40th Forum on Batteries, pp. 459xcx9c460 (1999).xe2x80x9d To improve the electroconductivity of a battery, it is necessary either to improve the dissociation characteristics of the supporting electrolyte to be used for the battery, or to select an organic solvent enabling the dissociation. characteristics of the supporting electrolyte to be improved. For the electrolyte, organic lithium salt is proposed as lithium salt excellent in dissociation characteristics over the lithium hexafluorophosphate (LiPF6) or lithium tetrafluoroborate (LiBF4) that is now mainly used. Above all, the lithium bis-(trifluoromethanesulfonyl)imide (LiN [SO2CF3]2) shown in Japanese Application Patent Laid-Open Publication No. Hei 05-326018 is known as a promising material having high solubility against a nonaqueous electrolytic solvent in comparison to an inorganic electrolyte. It is indicated, however, that the lithium bis-(trifluoromethanesulfonyl)imide (LiN [SO2CF3]2) has the drawback that it corrodes the aluminum used as the positive-electrode current collector for a secondary battery.
Also, organic lithium salt having boron to form its central ion and disalicylate to form its ligands, and organic lithium salt having boron to form its central ion and a benzenediolate derivative to form its ligands, are disclosed in Japanese Application Patent Laid-Open Publication No. Hei 07-65843. However, since these compounds have a benzene ring and are thus low in solubility, they cannot satisfy the electroconductivity or oxidation resistance required.
As disclosed in Japanese Application Patent Laid-Open Publication Nos. Hei 09-97627 and Hei 10-12272, to obtain noncombustibility or to ensure flameproofing, it is valid to use a fluorinated solvent. However, since the fluorine in fluorinated solvents is high in electron withdrawing ability, these solvents pose the problem that they deteriorate the electron releasing characteristics of their functional groups and, hence, the solubility and dissociation characteristics of lithium salt. Organic lithium salt has higher solubility than inorganic lithium salt, and is therefore a valid material.
For these reasons, the improvement of organic lithium salt is strongly desired as a promising material for improving the safety and performance of lithium secondary batteries.
The present invention is intended to supply: an organic borate compound from which a nonaqueous electrolyte high in electroconductivity can be created, and whose characteristics do not deteriorate even in a nonaqueous electrolyte having a noncombustible fluorinated solvent mixture; a lithium secondary battery using the organic borate compound; a nonaqueous electrolyte enabling the characteristics of an electrochemical capacitor to be improved; a long-life lithium secondary battery and an electric appliance, both using the nonaqueous electrolyte, and various applications of the electric appliance.
Electroconductivity is determined by the number of lithium ions and the mobility level thereof. Also, solubility is considered to exist within the radius of the ions surrounded by the solvent. For these reasons, the present inventors have considered it possible to suppress local coagulation of the solvent by increasing the anion of lithium salt dimensionally and delocalizing the electric charge of the ions, and to obtain highly dissociative and highly soluble lithium salt by introducing into the anion a functional group which can improve the affinity between the anion and the solvent and increase the electron withdrawing ability of the central element in the anion. Consequently, the inventors have energetically studied such a compound to complete the present invention.
That is to say, the present invention relates to an organic borate compound characterized in that it is represented by general formula (1) 
where X denotes lithium or quaternary ammonium or quaternary phosphonium and R1, R2, R3, and R4 each denote an independent halogen-atom displacement alkyl group whose carbon number ranges from 1 to 4.
The organic borate represented by general formula (1) above is a new compound excellent in solubility against a nonaqueous solvent, and the electrolytes using this compound are high in electroconductivity. More specifically, a halogen displacement acyloxy group having an excellent electron withdrawing ability and enabling an electric charge to be delocalized in a carbonyl group has been introduced into boron, the central element of the anion. Also, the ion radius of the anion has been increased and the electric charge of the ions has been delocalized. Consequently, it has been possible to improve solubility. For this reason, a nonflammable fluorinated solvent small in dipole moment and low in dielectric constant can also be applied to an electrolyte having such a supporting electrolytic property. Unlike imide-based organic salt, the compound described above does not corrode the current collector of aluminum, because the central ion of the compound is surrounded by ligands.
The organic borate compound in an embodiment of the present invention has the chemical structure represented as general formula 1, wherein X denotes lithium or quaternary ammonium quaternary phosphonium and R1, R2, R3, and R4 each denote an independent halogen-atom displacement alkyl group whose carbon number ranges from 1 to 4. In terms of molecular weight and oxidation stability, it is preferable that R1, R2, R3, and R4 are each a trifluoromethyl group or a pentafluoroethyl group.
A nonaqueous electrolyte that is very high in solubility against ring carbonate such as ethylene carbonate or propylene carbonate, against chain carbonate such as dimethyl carbonate or ethyl methyl carbonate, or against ether such as dimethoxyethane, and using each such solvent independently or in mixed form, can be obtained from the organic borate compound formed according to the present invention. In addition, a nonaqueous electrolyte high in electroconductivity can be obtained from either a compound using the fluorinated alkyl groups represented as R1, R2, R3, and R4 in general formula (1), or a compound using partially fluorinated alkyl groups, since these compounds have a solubility of at least 1.8 mol.dmxe2x88x923, even for a solution in which, in addition to the above-mentioned nonaqueous solvents, a nonflammable fluorinated solvent low in dielectric constant such as nonafluorobutyl methyl ether (tradename: HFE7100), is contained in the range from 5 to 90 volume percent. Furthermore, in addition to being wide in operating electric potential range because of its oxidation decomposition potential being high (about 5 V) against a lithium metal, the electrolyte obtained by dissolving such organic lithium borate does not corrode aluminum.
Examples of a compound which can be embodied using the organic borate pertaining to the present invention include: lithium tetrakis (trifluoroacetate) borate, lithium tetrakis (difluoroacetate) borate, lithium tetrakis (fluoroacetate) borate, lithium tetrakis (chlorodifluoroacetate) borate, lithium tetrakis (trichloroacetate) borate, lithium tetrakis (dichloroacetate) borate, lithium tetrakis (pentafluoropropanoate) borate, lithium tetrakis (3-chlorotetrafluoropropanoate) borate, lithium tetrakis (heptafluorobutanoate) borate, lithium tetrakis (2,2-bis-trifluoromethylbutanoate) borate, tetraethyl ammonium tetrakis (trifluoroacetate) borate, lithium tetrakis (difluoroacetate) borate, tetraethyl ammonium tetrakis (fluoroacetate) borate, tetraethyl ammonium tetrakis (chlorodifluoroacetate) borate, tetraethyl ammonium tetrakis (trichloroacetate) borate, tetraethyl ammonium tetrakis (pentafluoropropanoate) borate, tetraethyl ammonium tetrakis (3-chlorotetrafluoropropanoate) borate, tetraethyl ammonium tetrakis (heptafluorobutanoate) borate, tetraethyl ammonium tetrakis (2,2-bis-trifluoromethylbutanoate) borate,triethyl methyl ammonium tetrakis (trifluoroacetate) borate, tetraethyl phosphoniium tetrakis (trifluoroacetate) borate, triethl methyl phosphonium tetrakis (trifluoroacetate) borate etc.
Organic lithium borate based on the present invention can be synthesized using such a process as represented by, for example, the following formula: 
That is to say, organic lithium borate based on the present invention can be easily synthesized by generating reactions between boric acid, 3-quivalent acid anhydride, and lithium carbonate. However, the aforementioned process is one example of synthesizing the organic lithium borate pertaining to the present invention, and the synthesizing process is not limited to the aforementioned process. Also, organic borate ammonium salt or organic borate phosphonium salt can be obtained by using carboxylic acid quaternary ammonium salt or carboxylic acid quaternary phosphonium salt, instead of lithium carbonate.
A chain carbonate such as dimethyl carbonate, ethylmethyl carbonate, diethyl carbonate, dipropyl carbonate, or methylpropyl carbonate, a ring carbonate such as ethylene carbonate, propylene carbonate, butylene carbonate, triphloropropylene carbonate, or chloroethylene carbonate, vinylene carbonate, or dimethylvinylene carbonate, ring ester such as xcex3-butyrolactone or valerolactone, or chain ether such as 1,3-dioxysolan or tetrahydrofuran, can be used alone or in mixed form as the solvent for a nonaqueous electrolyte.
In addition to being used as the main supporting salt for a nonaqueous electrolyte, tetrahaloacetate borate based on the invention can be used as an additive, with LiPF6, LiBF4, LiN (SO2CF2CF3)2, LiN (SO2CF3)2, or the like, as the main supporting salt.
The positive electrode of the lithium secondary battery can use a lithium composite oxide containing a transition element capable of occluding and releasing lithium, such as cobalt, nickel, or manganese. The negative electrode can use a lithium metal capable of occluding and releasing lithium, graphite, an amorphous carbon material, silica, an oxide of tin, or a complex consisting of these substances and carbon.
The separator of the lithium secondary battery can use polyethylene, polypropylene, or a microstructured porous film laminate consisting of these substances.
As described in detail above, using tetravalent boron as the central element of the anion in organic lithium borate based on the present invention, and adjusting the electron withdrawing ability and molecular size of the functional group to be bonded to the boron improves solubility against a solvent small in dipole moment and low in dielectric constant, such as a nonflammable fluorinated solvent, and thus enables the improvement of the electroconductivity of this solution and the provision of a nonaqueous electrolyte valid for lithium secondary batteries and electrochemical capacitors. Also, problems associated with the prior organic lithium salts, namely, the corrosion of aluminum and the insufficiency in oxidation suppression potential, can be solved by using the above-mentioned organic lithium borate as a supporting electrolyte. In addition, since oxidation resistance improves, the high-temperature storage characteristics of lithium secondary batteries can be improved.
Since organic lithium borates based on the present invention can be dissolved to concentrations of at least 0.8 mol/dmxe2x88x923 in solvent mixtures heavily laden with a fluorinated solvent, in particular, it is possible to obtain nonaqueous electrolytes that offer a maximum electroconductivity value at a low concentration of 0.4 mol/dmxe2x88x923.
Furthermore, according to the present invention, lithium secondary batteries from consumer product-use ones to large-capacity ones intended for electric power storage and for electric automobile use can be essentially made nonflammable and thus the appropriate lithium secondary battery significantly improved in safety and high in reliability can be supplied according to a particular application. In addition, improvement in the safety of conventional lithium secondary batteries and reduction in the weight and size thereof are anticipated.
Besides, it is possible to attain the significant effect that a battery with internal or external protection circuits reduced in weight and size or without these circuits can be constructed. Also, since the noncombustibility of electrolytes based on the present invention alleviates quantitative restriction of the electrolytes which can be stored at their manufacturing site, greater quantities of electrolytes than at present can be stocked for manufacturing use and there is the merit that this effect will lead to more appropriate adjustment of manufacture and stock.